This invention relates to automatic cable equalization, and more particularly to an improved "walking limiter" type of automatic cable equalizer.
In a television studio environment, digital television signal equipment may be connected by cables with differing lengths and signal qualities as the systems and their interconnections are reconfigured for different applications. Since the digital signals carried by these cables degrade over longer distances, cable equalizers are used to compensate for these variations. These cable equalizers should be able to compensate for the losses created by 300 meters of cable, with a gain of 30 dB at the high frequencies needed for edge shaping and 6 dB at the lower frequencies of the NRZ (non-return-to-zero) data itself. For the common television standards of NTSC D2, PAL D2 and component D1, clock frequencies of 143 MHz, 177 MHz and 270 MHz, respectively, are involved. The NRZ data for each of these data rates has a theoretical minimum bandwidth of half of these frequencies.
One type of cable equalizer is known as a "walking limiter" equalizer. Such an equalizer comprises multiple equalizer stages. When a walking limiter cable equalizer is presented with an input signal that is degraded to the maximum capability of the equalizer, all of the stages operate linearly to restore the signal as much as possible. However, when a walking limiter cable equalizer is presented with an input signal that has more amplitude and full equalizing is not required, the stage or stages closest to the output go into limiting, so that the output signal is not amplified beyond the desired level.
In a cable equalizer with multiple equalizer stages, each of which clips the signal that it receives to the same, fixed, final level, problems arise in the presence of certain types of input signals. Referring to FIG. 1, if the incoming signal is of good quality, i.e., fairly fast rise times, but an amplitude below the clipping level, the first stage creates an overshoot as it sharpens the high-going transition. This overshoot is clipped, but as it trails off to the correct high level a premature trailing edge is produced. The high frequency gain of the subsequent stages then accentuates this premature trailing edge into a ringing that partially or completely closes the "eye" of the bit interval, causing errors.
One way to correct this problem is to precede the equalizer with an automatic gain control (AGC) amplifier. However, this leads to undesirable complexity and excessive power dissipation.